The present invention relates to the art of nuclear medicine and diagnostic imaging. It finds particular application in conjunction with 3D single photon emission computed tomography (SPECT), and will be described with particular reference thereto. It is to be appreciated that the present invention is amenable to other diagnostic modes and other like applications.
Diagnostic nuclear imaging is used to study a radio nuclide distribution in a subject. Typically, one or more radiopharmaceutical or radioisotopes are injected into a subject. The radiopharmaceutical are commonly injected into the subject""s bloodstream for imaging the circulatory system or for imaging specific organs which absorb the injected radiopharmaceutical. A gamma or scintillation camera detector head is placed near to a surface of the subject to monitor and record emitted radiation. Often, the detector head is rotated or indexed around the subject to monitor the emitted radiation from a plurality of directions. This data is reconstructed into a three-dimensional image representative of the radiopharmaceutical distribution within the subject.
Each detector head typically includes an array of photo multiplier tubes facing a large scintillation crystal. Each received radiation event generates a corresponding flash of light or scintillation that is detected by the photo multiplier tubes (PMT). Each photo multiplier tube that detects an event produces a pulse, pulses from tubes closest to the flash being bigger than pulses from further away tubes. The pulses from the individual PMT""s are combined to generate x and y spatial coordinates approximating the location of the scintillation event in the coordinate system of the crystal. Ambiguities arise as the distance between the tube and the subject increase. Long bore collimators mounted to the incident face of the camera are a known method to reduce these ambiguities.
Initially for SPECT, a single detector was mounted on a gantry to rotate about a subject. The orbit could consist of either 180 or 360 degrees of detector rotation. The application would dictate the necessary coverage angle. In the case of non-attenuation corrected SPECT imaging of the heart, it may be advantageous to perform 180 degree studies beginning at 45 degrees (assuming that 0 is below the table, counter clockwise rotation, and the subject is situated head first within the gantry). This and similar functioning 180 degree scans for other applications have been named the xe2x80x9cbest 180xe2x80x9d since the emission photons experience the least attenuation along with the least amount of collimator blurring.
The fundamental problem with SPECT distance blurring resides with the physics of collimating gamma photons. Collimating consists of placing a regularly spaced grids covering the detector field of view to limit receipt of photons to perpendicular or other fixed preselected rays. Two primary features of the collimator characterize the effects on photon event acceptance: collimator hole diameter and hole length. This hole-length combination defines a solid cone of acceptance for which any photon within this cone could potentially be counted. The larger the diameter of the cone at a specified distance, the greater the variance in incident ray angles and the poorer the spatial resolution of the resulting reconstructed image. As the cone diameter increases for a given distance, the more ambiguous the point of origin of the scintillation becomes.
When the best 180 is performed with a single detector, scan times are typically 40 minutes or more. To reduce total scan time, a second detector can be added positioned at 90 degrees to form an xe2x80x9cLxe2x80x9d. In this case, 180 degrees of data can be acquired in 90 degrees of gantry rotation, which cut the scan time in half. Given that the photon emission rates are adequate, this virtually allows the study rate to double. However, gamma camera detectors are surrounded by thick, lead shielded walls and have a useful field of view (UFOV) which is significantly narrower than the total detector width. In this case, dual detector xe2x80x9cLxe2x80x9d shaped detector configurations suffer the effect of placing one or both detectors at a sub-optimum distance from the patient. These dual xe2x80x9cLxe2x80x9d detectors have typically been configured in a fixed 90 degree orientation. This approach is not necessarily optimal with regards to performing the best 180. It suffers from physical limitations due to detector design which restricts the xe2x80x9creachxe2x80x9d or the distance from the detector physical edge to the useful field of view (UFOV) edge to typically 3.5-9.0 cm. That is, there is a 3.5-9 cm dead space in front of each detector head that can be received by only one detector. To avoid subject field of view truncation on the detector, subjects are typically positioned at the reach distance from both detectors. Undesirably, this position places the incident faces of both cameras 3.5-9 cm further from the subject than the optimal distance. Also in certain portions of the orbit, one of the detectors swings away from the subject. These two factors create extra distance between the detector and the subject which results in blurring even with collimation.
Some attempts have been made to reduce the distance between incident faces of cameras and the subject and resulting blurring such as cutting off corners to allow the detectors to fit closer together. Unfortunately, this reduces the shielding to the detector components while still not eliminating the gap between the two UFOV""s. Others have used long bore collimators to xe2x80x9cconnectxe2x80x9d the corners. Although this does reduce collimator blur, it dramatically reduces the detector sensitivity in an already count starved study type. Others have moved the table to keep the subject as close to the detector as possible. Unfortunately, this adds considerable complexity to manufacture (table robotics), and also places a high burden on preprocessing raw data to maintain a constant center of rotation. It further does not optimize the subject-detector distance in the region of interest since the detector edge to UFOV reach problem is still present. Moving the table during acquisition may also be disconcerting to some patients. Others have tried developing image processing and reconstruction techniques to reduce the blur inherent in collimated imaging. Some of these techniques produce unwanted reconstructed image artifacts or increased noise.
Most common cardiac studies performed today with the xe2x80x9cLxe2x80x9d shaped detector configuration incorporating a moving table to keep the patient in the UFOV. As previously stated, while this configuration only requires 90 degrees of detector rotation, it does not provide a minimum subject to detector distance as the detectors orbit the subject. Resulting blurred projection data causes a loss in reconstructed spatial resolution for which reliance upon post acquisition restoration techniques is placed.
The present invention contemplates a new and improved method and apparatus which overcomes the above-referenced problems and others.
In accordance with one embodiment of the present invention, a nuclear camera system includes a plurality of detector heads mounted around an examination region. A processor reconstructs signals from the detector heads into an image representation. Each detector head includes a housing which surrounds and shields a radiation sensitive incident face. A positioning mechanism is also included which selectively moves the detector heads with respect to the examination region to establish a relationship between the detector heads such that the housing for one head lies adjacent to and overlaps the housing for another detector head.
In accordance with another aspect of the present invention, the relationship between the detector heads changes or reverses at a selected point during a scan.
In accordance with another embodiment of the present invention, a method includes placing a first radiation detector housing adjacent to a second radiation detector housing with the incident faces of the radiation detectors toward the area of interest. The housing of one of the detectors overlaps the housing of the other detector. The method further includes rotating the detectors about a selected portion of the area of interest, and adjusting a position of the radiation detectors.
In accordance with another aspect of the present invention, the adjusting step includes reversing the overlap of the radiation detectors.
In accordance with another embodiment of the present invention, a method of diagnostic imaging includes placing an object of interest into an examination region observable to a plurality of detector heads and injecting a radioisotope into the object. A first detector head is then radially moved relative to the object of interest to minimize a distance between an incident face of the first detector head and the object of interest. A second detector head is tangentially moved to overlap the housing of the first detector head. The detector heads are then rotated partially about the examination region while receiving radiation events.
One advantage of the present invention resides in improved spatial resolution of reconstructed images.
Another advantage of the present invention resides in decreased reliance on post image acquisition processing techniques to reduce distance dependent blurring.
Another advantage resides in efficient data acquisition times.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.